A suppressing/enhancing effect of chloride, nitrate, and perchlorate on phosphate deposition was studied on aluminum in dilute sodium phosphate solutions under open-circuit conditions Despite weak adsorption of these anions on the oxidized aluminum surfaces, the deposition of phosphate in chloride-containing and chloride-free solutions is markedly different. While in the chloride-containing solution the deposition extended over several days and resulted in a large phosphate uptake, the deposition of phosphate in chloride-free solutions occurred within 1 h (or shorter) and produced only a thin oxide film. That is, some anions can `turn on' and some can `turn off' the multilayer phosphate deposition process. We believe that such a difference is due to the effect of the anions on aluminum dissolution. Chloride, by accelerating the release of aluminum ions from the metal through pitting corrosion, enhances the deposition of the phosphate-rich aluminum oxide. Perchlorate neither promotes nor inhibits the phosphate deposition, while nitrate, by mitigating the pitting corrosion, strongly retards the phosphate-rich film formation even in the presence of chloride ions. In this case, phosphate deposition proceeds via sorption rather than by a precipitation mechanism. The presented data can be read as evidence for a strong correlation between aluminum corrosion/corrosion inhibition and phosphate deposition processes.
ASJC Scopus subject areas
- Materials Science(all)